Surface treatment of magnesium and magnesium alloy articles



Patented Oct. 7, 1952 UNITED STATE simmer: TREAT ENT OF MAGNESIUM AND MAGNESIUM ALLOY ARTICLES,

Edward F. Fischer, Cleveland, Ohio, assignorto Aluminum Company of America, Pittsburgh, Pa, a c rrqra on of Pe sylvania N0. D awin Aprli ion July 3 1950. Serial No. 172,915;

1 This invention relates to the treatment of magnesium and magnesium base alloy articles to improve their appearance and resistance to corrosion and has special reference to providing an improved tarnish resisting surface and a pretreatment for the application of coatings. In

speaking of magnesium base alloys it is to be understood that this refers to alloys containing at least 70% by weight of magnesium.

Articles made of magnesium and'magnesium base alloys have enjoyed an increasing acceptance because of their satisfactory performance. This increased use has in part been the result of providing protective coatings and decorative finishes since in many places where the articles are employed the base articles would be' subject to corrosive attack or acquire an unsightly appearance. A large variety of coatings have been proposed, including those intended to serve as undercoats for paint, chemical'or electro-chemical finishes. One of the earliesttreatments which is still com monly used, frequently called the chrome pickle, consists of immersing articles in an aqueous solution containing nitric acid and sodi um dichromate in the proportions of 1.5 pints acid of 1.42 sp. gr. and 1.5 pounds of nazcrzommo in enough water to make 1 gallon. This leaves a varied colored film on the article which ranges in shade from a yellow to a yellow redgand in some cases produces a powdery y 6PQ i Although such a film. passivates the metal surface it does not provide prolonged protection against tarnish or corrosion and consequently is generally covered by another protective material suchas paint. The color and its non-uniformity over the entire surface of the article are handicap in ome applica ns espe ial Whe a bri ht fi sh is desired. Many efforts have been made, to obtain such a finish by chemical means and although the treatments proposed have increased the brightness of the articles, they have not pro- 3 duced a brightness similar to that obtained by buffing. Furthermore, they tend to remove an excessive amount of metal. The expression, bright finish, as. used below, refers to a lustrous surface having a high reflective quality. In many cases, depending on the metal and its. sur-. a condition, th br gh fini h. may po sess a hi h d ree of spssul rity.

In order to provide satisfactory coatings on magnesium and m g sium base a loy articles, it ha been ner lly n c ssary to prepare the surface by one r m re of such preliminary treatments as degreasing, mechanical abrasion or chemical attack. Since the surface of the articles 10. Claims. (01. 14s-..e.2)

varies according to the method by which they have been fabricated such as by rolling, forging, extrusion or casting, the choice of the preliminary treatment will be determined to some extent by the nature of the-surface. as well as by its cost and effectiveness. Thus, in treating die castings, for example, it has been found that a mechanical cleaning operation is generally required to remove the surface film or skin produced by the casting process. Such surface preparation is relatively expensive but no satisfactory substitute has found commercial acceptance.

It is an object of this invention to provide a chemical treatment for magnesium and magneslum base alloy articles which produces a uniform bright finish over the entire surface of the article. Another object is to provide a bright finish which is more resistant to tarnishing than those available heretofore. A further object is to provide a chemical treatment which is suitable for removing the surface film from castings. Still another object is to provide a chemical treatment that is capable of producing a specular surface without the use of an electric current or mechanical bufling. These and other objects will be apparent in the following description of the inven tion. l

I have discovered that a greatly improved bright finish that is highly resistant to tarnishing or loss of brightness is produced on magnesium and magnesium alloyarticles by simply immersing them for a short time in an aqueous solution containing chromic, nitric and hydrofluoric acids in certain proportions. Also, less metal is" removed than generally occurs upon contact with the acid solutions employed heretofore for surface finish-purposes. Forconvenience this solu-' tion is referred to herein as the bright dip solution. Qn articles made of magnesium or alloys containing little or no aluminum, or having 'a homogeneous microstr-ucture, a specular finish is developed by exposure to the solution. On articles made of alloys containing larger amounts of aluminum or a substantial quantity of other elements which produce a heterogeneous micro structure, the solution produces a uniform bright silvery semi-matte finish, Although the specular quality of an article is most striking if the surface is. smcc h. .I. have. und. that the bright d p so ut nn a produces. a very pleasing appearance on castin s th h v a r gh iace.

The bri ht finishes produc d as d ribed are hi ly r si ta o tarnish g upon-long exposure to the air, being more, durable in this respect than bright finishes obtainable with present commercial treatments. To preservethe original appearance indefinitely, the article may be coated with any of the well known clear lacquers.

Finishes produced with the type of solution referred to above provide an excellent uniform surface for subsequent surface treatments. For example, it has been possible to form a uniform black coating on die castings by first treating them in the bright dip solution and then immersing them in one of the conventional solutions for producing black coatings on magnesium and its alloys. An aqueous solution of the type which contains potassium chrome alum and so dium dichromate is quite satisfactory, one containing 3% of the alum and 10% of the dichromate being preferred. The uniform finish can be obtained without the customary preliminary buifing operation to remove the common surface film or skin occurring in die castings.

An aqueous solution capable of producing the results referred to above must-contain as the essential components thereof, one, weight basis, from to 60% of chromic acid (C1O3), 0.01 to 2% of hydrofluoric acid (HF) and 0.75 to 12% of nitric acid (HNOa). Within the foregoing ranges the nitric acid should be between 1.25 and 75% of the chromic acid content. For optimum results the solution should contain from 18 to 35% of chromic acid, from 0.04 to 1% of hydrofluoric acid and 1 to 5% of nitric acid, and the nitric acid should amount to 2.8% to 28% of the chromic acid content. For chromic acid concentrations on the order of 50%, and higher, the nitric acid content of the solution should be less than 3% and the hydrofluoric acid content should be less than 0.2%. Other substances may be added to the solution provided they have no substantial subversive effect upon the bright finish. For example, certain inorganic salts may be helpful in retarding or promoting the action of the acid solution without substantially adversely affecting the desired finish. Of course, some magnesium salts, and possibly salts of other metals, accumulate with continued use of the acid bath. In time it may be necessary to replenish one or more of the acid components to compensate for. any losses. However, the production of the bright finish depends on the presence of the acid components named above within the stated ranges and the avoidance 'of other substances which substantially interfere with obtaining a bright finish.

The proportions of the three acids given above are essential to the production of a bright finish. Unless the minimum ratio of nitric to chromic acid (lowest amount of nitric with respect to chromic acid) is observed, the finish will not be bright even though hydrofluoric acid is present. On the other hand, if the maximum ratio (largest amount of nitric with respect to chromic acid) is exceeded, a dull colored finish is obtained. In addition to observing the proper ratio between chromic and nitric acids at least 0.01% hydrofluoric acid is needed to obtain the desired result but more than 2% causes the finish to become dull.

To obtain the best results the solution temperature should be maintained within the temperature range of to F. It is preferable to use a temperature within the upper portion of this range, that is, above 85 F. when treating alloys having the aluminum content above about 5%. This is especially true for the treatment of die castings. For alloys containing less than 5% aluminum, the temperature of the solution should be no higher than 75 F.

ment.

The period of immersion will vary with cornposition of the metal being treated, some alloys, for example, requiring a longer immersion period than others. The temperature of the solution also affects the length of immersion, shorter periods generally being possible when the temperature is in the upper portion of the range mentioned above. Generally, a period of at least minute is required While the maximum pe riod is 5 minutes. Longer periods of exposure to the acid solution do not produce a better finish. I prefer to immerse the articles for a period of to 2 minutes.

One of the advantages gained from using my acid solution is that the desired finish is obtained by immersing the articles in but one solution. In other surface treating processes it is frequently necessary to employ two or more solutions with attendant increase in cost and time.

Although the magnesium or magnesium alloy articles may be immersed in the bright dip without any previous treatment, it is advisable to clean them thoroughly before dipping. This may be done in any of the well known manners. One of the preferred preliminary treatments consists in subjecting the article to an aqueous 10% caustic soda solution maintained at a temperature between about F. and the boiling point of the solution followed by rinsing in cold water.

Under some conditions the bright dip solution may leave a slight color in the surface finish. I have discovered that this coloration can be removed by immersing the articles in a hot aqueous solution containing by weight of 1 to 20% alkali metal hydroxide for a period of to 2 minutes after the bright dip treatment.v The articles, of course, should be rinsed upon removal from the bright dip solution and before immersion in the hot hydroxide solution. The latter treatment is especially useful in improving the finish on die castings and those alloys containing more than 5% aluminum. The solution should be maintained at a temperature between 180 F. and its boiling point during the treat- Of the alkali metal hydroxides, those of sodium and potassium are preferred in amounts of 5 to 10%. If the articles are to be subsequently colored they are withdrawn from the hydroxide solution and then immersed in a coloring solution such as that described hereinabove.

Articles made of any of the commercial magnesium base alloys may be treated in the bright dip solution with beneficial results. However, I have observed that the highest specular finish is obtained on articles of commercially pure magnesium or those alloys containing less than about 5% aluminum with or without other minor amounts of alloyin elements and which have a substantially homogeneous microstructure. Bright semi-matte finishes can be obtained on alloys containing as much as 12% aluminum with or without other alloying elements even though the microstructure is not homogeneous. The solution can, of course, be advantageously employed in treating alloy articles which contain no aluminum.

The improved finish obtainable through use of my invention is illustrated in the following comparative tests. In these tests Solution #1 was prepared according to the present invention. Solution #2 represented a commercial solution hitherto employed for producing What was considered to be a bright surface, and Solution #3 typified a composition said to provide a superior Soluticuit? Per cent Nitric acid nNo3 Sulphuric. acid (I-12304 Potassium dichromate (Kzc'r zO qiul Q16 Potassium fluoride (KF) 1.5 T e test pa els used were cold rolled magnesium alloy sheet 0.064 inch in. thickness. The magnesium alloy had a nominalcomposition of, 3% aluminum, 1% zinc, 0.2% manganese and balance. magnesium. Three panels wereseparately immersed in each solution for a period of 2 minutes, the solutions being maintained at a temperature of 70-75 F. Thefpanels were rinsed and dried after the treatment in the solutions. Those which had been immersed in Solution #1 had by far the highest specular reflectivity. Ifhe panels treated in Solution #2 had awhitened, rough, non-uniform appearance. Those panels exposed to Solution #3 had the characteristic chrome pickle color. These differences were confirmed by quantitative measurements on a stand! ard Hunter Multipurpose Refiectometer which is described in the National Bureau of Standards Journal of Research, vol. 25, pages 581.618, No-J vember 1940. Readings were takenon both sides of each panel and an average value was thus obtained which represented the effect of each solution. The luminous apparent reflectance, specular gloss and contrast gloss, valueswere thus determined as compared to vstandard'speci men having a specular gloss value .of 8.00 and a contrast gloss value of 47.2. The average results are as follows:

Specular reflectivity of test panels' Luminous Specular Contrast Solution No. Apparent Y Reflectance Gloss Gloss The magnitude of the value for Solution #1 clearly reveal the very marked superior specular reflectivity of the test panels. These results are typical of many I have observed. The panels inimersed in Solutions #2 and #3 had a dull appearance. 1 I

The effect of varying the proportions of chromic, nitric and. hydrofluori acids upon the bri htness and color or the finish is'illustrated in the following tests. Test panels ofthe same magnesium base alloy were used as in the preceding tests. The panels were immersed in the solutions having the compositions listed below for a period of 1 minute, washed and dried.- The solutions were maintained at a tempe'rature'of W5 F. The composition of the solutions and the appearance of the panels treated therein are given in the table below.

cent

il cble Efi cct of carvin l tiflzl ttions stores.

"'HNOaand HF v 7 Solution Percent Percent Percent I Nor 7 BN0? 7, V F Appearance of Panel i054 -1.4 0.025 Bright. 1

58. 4 l. 61 0. 08 D0. 22.0 4.5 0.9 Very bright. 20.2 I 10.6 1.9 Bright. 20.1 10.5 2.2 Colored, non-uniform, Y localized roughening and whitening.

These results and those obtained with Solution #1 previously described show that a bright flnishjca be obtained within the rangev of solution composition defined above. In particular it to be noted that an HF content of 2.2% produced inferior results.

This application is a continuation-impart of my oo-jpending application Serial No. 761,168, filedJuly 15, 1947,;now abandoned.

Having thus described my invention and certain embodiments thereof, I claim:

1. The method of treating artielesof magnesium, and magnesiumbase alloy-s to produce thereon a bright finish ranging from silvery semimatte'to specular, saidmethod comprising immersing said articlesfor at least A; minute in an aqueous'solution having as essential components thereof for producing'said'bright finish on said metal articles 0.75 to 12 by weight of nitric acid, 5 to by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the Weight of the nitric acid is between 1.25 and of the weight of the chromic acid, saidratio diminishing as the chromic acid content increases beyond the point where th minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration 01-50% and more of chromic acid the nitric acid doesnotexceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the-solution. I I

2. The methodof treating articles of magnesium and magnesium base alloys to produce thereon a'bright finish ranging from silvery semimatte to specular,- said method comprisin immersing said'articlesfor at least minute in an aqueous solution having as essential components thereof for producing said bright finish on said metal articles 1 to 5% by weight of nitric acid, 18 to 35% by weight of chromic acid, the amount of nitric acid present with respect to chromic acid being such that theweight of the nitric acid is between 2.8 and 28% of the weight of the chromic acid, and 0.04 to 1 by weight of hydrofluoric acid. v

3. The method of treating articles of magnesium and magnesium base alloys to produce thereon a bright finishranging from silvery semimatte to specular, said method comprising immersing said articles for at least minute in an aqueous solution having as essential components thereof for producing said bright finish on said metal articles 0.75 to 12% by weight or nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of the nitric acid is between 1.25 and 75% of the weight of the chromic acid, said ratio diminishing as the chromic acid content increases beyond the point Where he minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50 and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by Weightof the solution, said solution being maintained at a, temperature between 60 and 100 F.

4. The method of producing a highly specular reflective surf-ace on articles and of magnesium and magnesium alloys containing less than aluminum, said method comprising immersing said articles for a period of to 2 minutes in an aqueous solution having as essential components thereof for producing said specular surface 0.75 to 12% by weight of nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being. such that the weight of the nitric acid is between 1.25 and 75% of the weight of chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% andmore of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of thev solution, said solution being maintained at a temperature between 60 and 75 F. i

5. In the process of applying a black color to magnesium base alloy. die castings, the steps comprising immersing said castings for a period of A; to 5 minutes in an aqueous solution having as essential components thereof for producing a bright finish thereon, prior to the application of the black color, 0.75 to 12% by weight of nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of the nitric acid is between 1.25 and 75% of the weight of the chromic acid, said ratio diminishing as the chromic acid content increases beyond the point wher the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the solution, said solution being maintained at a temperature between 85 and100 F., withdrawin said castings from said solution and thereafter immersing them in an aqueous solution containing potassium chrome alum and sodium dichromate as the essential components to produce a black color on said castings.

6. The method of producing a bright finish ranging from silvery semi-matte to specular on magnesium and magnesium base alloy articles comprising cleaning said articles in a hot aqueous caustic soda solution, withdrawing said articles from said solution, rinsing them and thereafter immersing them for a period of at least minute in an aqueous solution having as essential components thereof for producing said bright finish on said articles 0.75 to 12% by weight of nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of nitric acid is between 1.25 and 75% of the weight of chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed0.2% by weight of the solution.

7. In the art of surface treating articles of magnesium and magnesium base alloys the steps in preparation for a final finishin treatment comprising immersing said articles for a period of at1east minute in an aqueous solution having as essential components thereof for improving the metal surface, 0.75 to 12% by Weight of nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of nitric acid is between 1.25 and of the weight of the chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can b used with the maximum amount of nitric acid so that at a, concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the solution, removing said articles from said solution and rinsing.

' 8. The method of treating articles of magnesium and magnesium base alloys to produce thereon a bright finish ranging from silvery semimatte to specular, said method comprising immersing said articles for at least minute in an aqueous solution having as essential components thereof for producing said bright finish on said articles 0.75 to 12% by weight of nitric acid, 5 to 60% by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of nitric acid is 1.25 to 75% of the weight of chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the solution, withdrawing said articles from said solution and thereafter immersing them in a sec ond aqueous solution containing from 1 to 20% of an alkali metal hydroxide maintained at a temperature between F. and the boiling point of said solution.

9. The method of treating articles of magnesium and magnesium base alloys to produce thereon a bright finish ranging from silvery semimatte to specular, said method comprising immersing said articles for at least minute in an aqueous solution having as essential components thereof for producing said bright finish on said articles 0.75 to 2% by weight of nitric acid and 0.01 to 2% by weight of hydrofluoric acid, 5 to 60% by weight of chromic acid, the ratio of nitric acid with respect to chromic acid being such that the weight of nitric acid is 1.25 to 75% of the weight of chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the solution, withdrawing said articles from said solution and thereafter immersing them in a second aqueous solution containing from 5 to 10% of at least one of the hydroxides of the group composed of sodium and potassium hydroxide, said second sor 9 lution being maintained at a temperature between 180 F. and the boiling point of the solution.

10. In the process of applying a black color to magnesium base alloy die castings, the steps comprising immersing said castings for a period of A to 5 minutes in an aqueous solution having as essential components thereof for producing a bright finish on said castings, prior to the application of the black color, 0.75 to 12% by weight of nitric acid, 5 to by weight of chromic acid and 0.01 to 2% by weight of hydrofluoric acid, the ratio of nitric acid with respect to chromic acid being such that the weight of nitric acid is between 1.25 and of the weight of chromic acid, said ratio diminishing as the chromic acid content increases beyond the point where the minimum amount of chromic acid can be used with the maximum amount of nitric acid so that at a concentration of 50% and more of chromic acid the nitric acid does not exceed 3% by weight and the hydrofluoric acid does not exceed 0.2% by weight of the solution, said solution being maintained at a temperature between and F., withdrawing said castings from said solution, rinsing them, immersing them for a period REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,138,794 Nelson Nov. 29, 1938 2,186,579 Dnbpernell Jan. 9, 1942 2,428,749 De Long Oct. 7,1947

FOREIGN PATENTS Number Country Date 229,616 Switzerland Feb. 1, 1941 

10. IN THE PROCESS OF APPLYING A BLACK COLOR TO MAGNESIUM BASE ALLOY DIE CASTINGS, THE STEPS COMPRISING IMMERSING SAID CASTINGS FOR A PERIOD OF 1/4 TO 5 MINUTES IN AN AQUEOUS SOLUTION HAVING AS ESSENTIAL COMPONENTS THEREOF FOR PRODUCING A BRIGHT FINISH ON SAID CASTINGS, PRIOR TO THE APPLICATION OF THE BLACK COLOR. 0.75 TO 12% BY WEIGHT OF NITRIC ACID, 5 TO 60% BY WEIGHT OF CHROMIC ACID AND 0.01 TO 20% BY WEIGHT OF HYDROFLUORIC ACID, THE RATIO OF NITRIC ACID WITH RESPECT TO CHROMIC ACID BEING SUCH THAT THE WEIGHT OF HYDROFLUORIC ACID BETWEEN 1.25 AND 75% OF THE WEIGHT OF CHROMIC ACID, SAID RATIO DIMINISHING AS THE CHROMIC ACID CONTENT INCREASES BEYOND THE POINT WHERE THE MINIMUM AMOUNT OF CHROMIC ACID CAN BE USED WITH THE MAXIMUM AMOUNT OF NITRIC ACID SO THAT AT A CONCENTRATION OF 50% AND MORE OF CHROMIC ACID THE NITRIC ACID DOES NOT EXCEED 3% BY WEIGHT AND THE HYDROFLUORIC ACID DOES NOT EXCEED 0.2% BY WEIGHT OF THE SOLUTION, SAID SOLUTION BEING MAINTAINED AT A TEMPERATURE BETWEEN 85 AND 100* F., WITHDRAWING SAID CASTINGS FROM SAID SOLUTION, RINSING THEM, IMMERSING THEM FOR A PERIOD OF 1/2 TO 2 MINUTES IN A SECOND AQUEOUS SOLUTION CONTAINING FROM 1 TO 20% BY WEIGHT OF AN ALKALI METAL HYDROXIDE MAINTAINED AT A TEMPERATURE BETWEEN 180* F. AND THE BOILING POINT OF SAID SOLUTION, WITHDRAWING SAID CASTINGS FROM SAID SECOND SOLUTION, RINSING THEM AND THEREAFTER IMMERSING THEM IN A THIRD AQUEOUS SOLUTION CONTAINING POTASSIUM CHROME ALUM AND SODIUM DICHROMATE AS THE ESSENTIAL COMPONENTS TO PRODUCE A BLACK COLOR ON SAID CASTINGS. 